Quarter onions lengthwise, then thinly slice crosswise. Heat 2 tablespoons oil in a soup pot or Dutch oven over medium-high heat. Add the onions and 1/4 teaspoon salt; cook, stirring frequently, until the onions are soft and beginning to brown, 6 to 8 minutes. Stir in green curry paste and cook, stirring, for 3 minutes. Stir in 4 cups broth; bring to a gentle simmer.
Meanwhile, coarsely chop 4 cups spinach. Combine the remaining 4 cups spinach with water in a blender; puree until the spinach is chopped to confetti. Cut mushrooms into 1/4-inch strips.
Heat the remaining 2 teaspoons oil in a large skillet over medium heat. Add garlic and cook, stirring until fragrant, about 30 seconds. Add the mushrooms and cook, stirring, until the liquid cooks away and the mushrooms begin to colour, 4 to 6 minutes.
Stir the mushroom mixture and green beans into the pot; return to a simmer and cook for 5 minutes. Stir in broccoli stems, scallions and lemongrass; return to a simmer and cook for 3 minutes more. Stir in the chopped and pureed spinach and cilantro. Return to a simmer, cover and cook just long enough to wilt the spinach, about 1 minute. Add up to 1 cup additional broth if a thinner consistency is desired. Add lemon juice; taste and add more salt, serrano and/or lemon juice, if desired
*To prepare lemon grass: Trim off the root end and grassy top. Peel off the woody outer leaves. Thinly slice the softer inner stalk, then finely chop.